As surface-treated steel sheets for use in cans, tin-plated steel sheets referred to as “tinplates” have been widely used. Generally in such tin-plated steel sheets, a chromate coating is formed on the tin-plated surface of the steel sheets by chromate treatment such as immersing the steel sheet in an aqueous solution containing a hexavalent chromium compound such as dichromic acid, or electrolyzing the steel sheet in the solution. This is because, by formation of the chromate coating, oxidation of the tin-plated surface that is likely to occur due to long-term storage or the like can be prevented, and a degradation of appearance (yellowing) can be suppressed. In addition, when lacquer is applied to the tin-plated steel sheet before use, cohesive failure due to the growth of a tin (Sn) oxide layer is prevented and adhesion with organic resin such as paints, (hereinafter simply referred to as “paint adhesion”) is ensured.
In contrast, considering recent environmental problems, restriction of the use of chromium has proceeded in various fields, and some chemical conversion treatment techniques in stead of the chromate treatment have been proposed also for the tin-plated steel sheets for cans.
For example, Japanese Examined Patent Application Publication No. 55-24516 discloses a method for surface-treating a tin-plated steel sheet. The method includes forming a chemical conversion coating by performing direct current electrolysis using the tin-plated steel sheet as a cathode in a phosphoric acid solution. Japanese Examined Patent Application Publication No. 58-41352 discloses a chemical conversion solution containing phosphate ions, one or more of chlorates and bromates, and tin ions and having a pH of 3 to 6. Japanese Unexamined Patent Application Publication No. 49-28539 discloses a surface treatment method, for tinplates, including applying one or more of calcium phosphates, magnesium phosphates, and aluminum phosphates so that the coating thickness is 15 μg/m2 or lower. Japanese Unexamined Patent Application Publication No. 2005-29808 discloses a surface-treated steel sheet, for containers, successively having an iron (Fe)-nickel (Ni) diffusion layer, an Ni layer, and an Ni—Sn alloy layer, a non-alloyed Sn layer and further having 1 to 100 mg/m2 of a phosphate coating layer in terms of phosphorus (P) on the steel sheet surface.
However, the chemical conversion coatings disclosed in JP '516, JP '352, JP '539 and JP '808 cannot suppress degradation of appearance or reduction in paint adhesion caused by oxidization of the tin-plated surface compared to conventional chromate coatings.
In contrast, Japanese Unexamined Patent Application Publication No. 2007-239091 discloses a method for manufacturing a tin-plated steel sheet including plating a steel sheet with tin, immersing the tin-plated steel sheet in a chemical conversion solution containing tin ions and phosphate ions or subjecting the steel sheet to cathodic electrolysis in a chemical conversion solution, and then heating the same to 60 to 200° C. to form a chemical conversion coating, thereby suppressing degradation of appearance and reduction in paint adhesion caused by oxidization of the tin-plated surface to a degree equal to or higher than the suppression degree obtained by conventional chromate coatings.
However, the method disclosed in JP '091 has a problem that a heating unit used subsequently to chemical conversion is necessary and therefore the cost of chemical conversion is high.
It could therefore be helpful to provide a tin-plated steel sheet, without using Cr, that can suppress degradation of appearance and reduction in paint adhesion caused by oxidization of the tin-plated surface and can be subjected to chemical conversion treatment at low cost and a method for manufacturing the same.